1. Field of the Invention
The present invention relates to a sensor IC, a contact image sensor and an image reading apparatus for reading color documents.
2. Description of the Related Art
Sensor ICs for reading color documents have conventionally been proposed (see, e.g. JP-2006-332250). FIG. 28 schematically illustrates a light receiving portion 90 of a conventional sensor IC. The light receiving portion 90 shown in the figure includes three light-receiving-element rows 91, 92 and 93. Each of the light-receiving-element rows 91, 92, 93 has a plurality of light-receiving elements 911, 921, or 931 aligned in the primary scanning direction x. These rows are provided with color filters of three primary colors, red, green and blue, respectively. Specifically, the light-receiving-element row 91 is provided with a color filter that passes red light, the light-receiving-element row 92 is provided with a color filter that passes green light, and the light-receiving-element row 93 is provided with a color filter that passes blue light. The sensor IC having this structure is used as incorporated in e.g. a contact image sensor. The contact image sensor further includes a light source for illuminating a document and a lens array for guiding the light reflected by the document to the light receiving portion 90. The contact image sensor simultaneously reads three colors by scanning while illuminating the document by the light source.
In the conventional sensor IC, the width of the light receiving portion 90 in the secondary scanning direction y is large because the light-receiving-element rows 91, 92 and 93 are arranged parallel to each other. For instance, when compared with a light receiving portion including a single light-receiving-element row, the width of the light receiving portion 90 is about five times as large as that of the light receiving portion having a single light-receiving-element row. To uniformly illuminate a region corresponding to the light receiving portion 90, a large amount of light needs to be emitted from the light source, and variations of the light amount in the secondary scanning direction y needs to be avoided. Since the lens array has a difference in index of refraction between a position close to the optical axis and a position distant from the optical axis, passing the light having a large width in the secondary scanning direction y hinders enhancement of reading quality.
In recent years, the reading of light outside the visible light range, such as infrared light, is sometimes demanded in addition to the reading of visible light. In such a case, the width of the light receiving portion 90 tends to increase, which makes the above-described problem more serious.